HR: 17:00h
AN: H24C-05    [Abstracts]
TI: Regional and Local Control of Arsenic Concentrations in Shallow Aquifers by the Permeability of Surface Soils
AU: * van Geen, A
EM: avangeen@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964 United States
AU: Aziz, Z
EM: aziz@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964 United States
AU: Goodbred, S L
EM: sgoodbred@notes.cc.sunysb.edu
AF: Marine Sciences Research Center, Stony Brook University, Stony Brook, NY 11794 United States
AU: Zheng, Y
EM: yzheng@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964 United States
AU: Horneman, A
EM: horneman@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964 United States
AU: Dhar, R
EM: dhar@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964 United States
AU: Weinman, B
EM: bweinman@ic.sunysb.edu
AF: Marine Sciences Research Center, Stony Brook University, Stony Brook, NY 11794 United States
AU: Cheng, Z
EM: czhongqi@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964 United States
AU: Stute, M
EM: martins@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964 United States
AU: Hoque, M A
EM: hoques@agni1.net
AF: Geology Department, University of Dhaka, Dhaka, 1000 Bangladesh
AU: Seddique, A A
EM: aseddique@yahoo.com
AF: Geology Department, University of Dhaka, Dhaka, 1000 Bangladesh
AU: Ahmed, K M
EM: kmahmed@udhaka.net
AF: Geology Department, University of Dhaka, Dhaka, 1000 Bangladesh
AB: One of the bewildering aspects of the current arsenic crisis in Bangladesh and other South Asian countries is the extreme degree of spatial variability of groundwater As concentrations. This presentation focuses on the origin of this variability in the top 20 meters of shallow aquifers by combining surface geophysical measurements (EM31) with groundwater and sediment properties obtained by modifying a local hand-drilling method in three contrasting areas of Bangladesh: (1) Birganj, in northwestern Bangladesh, where groundwater As concentrations rarely exceed 50 ug/L, (2) Araihazar, a central portion of the country where shallow groundwater As concentrations are highly variable, and (3) Lakshmipur, where essentially all shallow wells are elevated in As. Comparison with a series of auger cores collected in Araihazar indicates that the EM31 signal combines contributions related to the ionic strength of soil water as well as the proportion of fine-grained sediment. The combined set of observations shows a rather consistent relation between the conductivity of surface soils measured by induction and shallow groundwater As. In Birganj, EM31 conductivities rarely exceed 10 mS/m and can be below the detection limit of the instrument (~0.1 mS/m) over distances of 100s of meters. In Araihazar, areas with EM31 conductivities ranging form 10-15 mS/m are intermixed with regions with EM31 readings of 20-30 mS/m. The lower and higher EM31 conductivity ranges are generally associated with low and elevated As concentrations, respectively. In contrast, EM31 readings in the portion of Lakshmipur that was surveyed are consistently high and range from 30-50 mS/m. Overall, therefore, it appears that high groundwater As concentrations are typically associated with aquifers capped by fine-grained sediment whereas low groundwater As concentrations prevail in aquifers overlain by sandy deposits. This association, combined with gradual downstream fining of surface deposits of the Ganges-Brahmputra-Meghna delta, helps explain the spatial variability of shallow groundwater As variations down to the ~1 km scale (i.e. within Araihazar) as well as broader patterns throughout the country at the ~100 km scale.
DE: 1831 Groundwater quality
DE: 1045 Low-temperature geochemistry
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting